1. Field of the Invention
This invention relates to replenishing photographic silver halide developer solutions. More particularly, this invention relates to replenishing the developer used in automatic processors for lithographic film.
2. Description of the Prior Art
Developer solutions used in automatic photographic film processors are subject to change in composition due to accumulation of development products, e.g., oxidized developer and halide ion, and to developer aging. Such changes are particularly critical in processing lithographic film where the hydroquinone type developer is characterized by high sensitivity to bromide and sulfite ion concentration and where developer stability is characterized by high sensitivity to pH, to aerial oxidation, and to anaerobic degradation. The nature of lithographic development, developers and replenishers has been characterized in prior art studies such as J. A. C. Yule, J. Franklin Inst., 239, 221 (1945); L. F. A. Mason, "Photographic Processing Chemistry", The Focal Press, New York, (1966) pg. 163; D. A. Pullen & M. C. Lloyd, Printing Technology, 14, 69 (1970); L. T. Connolly, TAGA Proceedings, 1970 page 151; R. L. Childers, Photogr. Sci. Eng., 15, 480 (1971); and H. Zwicky, Chimia, 26, 667 (1972). In a commercial lithographic developer hydroquinone is the sole developing agent. To obtain the lith effect with consistent results, the developer must have a low, but constant, sulfite ion concentration along with a constant high pH and bromide ion concentration. The sulfite ion concentration is maintained by a sulfite buffer, e.g., sodium formaldehyde bisulfite; during use, however, concentrations of hydroquinone and bromide ion and pH are changed by the development process. Due to the low sulfite concentration and to the presence of buffer components, lith developer at high pH (i.e., substantially above 8) undergoes a time-dependent loss of activity due to aerial oxidation and anaerobic degradation. Thus to obtain reproducible processing, a replenisher solution is normally added to the developer to compensate for changed activity due to prior use or aging. Replenisher, however, contains substantially the same components as developer itself, and consequently is unstable. This poses the problem of attempting to compensate for the deteriorating activity of the developer with a replenisher solution whose activity is also deteriorating. In addition to maintaining developer activity, the concentration of bromide ion in the developer, which modulates development, should be maintained constant. In some automatic processors, means are provided for replenishing the developer periodically by measuring or sensing the activity level of the developer bath and then adding the appropriate amount of replenisher. OLS No. 2,119,069 discloses an automatic developer replenishing system which monitors bromide ion and developing agent concentration and adds, as required, either replenisher with high bromide concentration or replenisher with no or low bromide concentration. Schumacher, OLS No. 2,004,893 discloses a replenishment system containing two separate replenishers in which one replenisher is supplied to the developer on the basis of the quantity and developer requirements of the film being developed, and in which a second replenisher is supplied to the developer on the basis of processor running time. Automatic replenishing systems, such as that of OLS No. 2,119,069, are normally expensive and use premixed replenishers, which, like the developer, are unstable. Consequently, replenisher activity is never the same from one moment to the next, and the replenisher is frequently discarded if not used within a relatively short period of time.